This invention relates to a method and apparatus for corrosion monitoring during equipment cleaning. More particularly, this invention relates to a method and apparatus for monitoring corrosion during cleaning of a steam generator, particularly in a nuclear power plant. Still more particularly, this invention relates to a self-sampling monitor for use during steam generator cleaning to measure corrosion due to cleaning solvents in the equipment being cleaned. Still further, this invention relates to an improved automated linear polarization corrosion monitor for use with the self-sampling monitor mentioned above.
A number of processes are known in which corrosion of the equipment caused by process parameters is of concern for reasons including cost of capital equipment, redundancy design, maintenance, safety, and process efficiency. Thus, a number of techniques have been developed over a long period of time for monitoring corrosion in process equipment. One typical method involves the use of a test monitoring coupon which is strategically placed at a location within the process which is considered to be representative of the corrosion occurring in the system being observed. Measurement of the corrosion of the test monitoring coupon thus provides an indication of the status of the corrosion.
In some processes which are not significantly corrosive during normal operation, the most corrosive event for the equipment can occur during cleaning with solvents during equipment shutdown. For example, for steam generators used in producing power, such as in nuclear power plants, a shell and tube type heat exchanger is periodically shut down to remove deposits which occur due to a number of factors, such as the purity of the water, the corrosion of equipment in the entire system which might produce, for example, iron oxide deposits, and the like. During shut down, the steam generator is chemically cleaned by using one or more solvents. One type of steam generator cleaning uses an external heating and solvent recirculation system to maintain solvent temperature during cleaning. The preferred placement of corrosion monitoring coupons and electrodes during chemical cleaning is within the steam generator.
It is often an aim in this art to use, when beneficial, the primary system heat during steam generator chemical cleaning instead of an external heating and solvent recirculation system to maintain solvent temperature. Such a technique would concentrate heating at the tubes and tube crevices, which are difficult to clean, could save time for cleaning by reducing the cool down time necessary to initiate cleaning, and could reduce equipment costs. Thus, such a technique is attractive for use in steam generator cleaning. However, such a technique presents novel challenges for corrosion monitoring. Since the steam generators are not drained or cooled to below approximately 100.degree. C. before cleaning, it is not possible to open the steam generators and install corrosion surveillance equipment before the cleaning. It is not even feasible to make connections directly to the steam generator handholes or other penetrations into the equipment. Instead, connections must be made to permanent isolatable piping systems connected to the steam generator, such as the blowdown piping system for the generator. Connecting a corrosion monitoring vessel to the steam generator in this way causes novel problems. For example, in the time it takes for the chemical cleaning solvent to traverse the blowdown piping and arrive at a corrosion monitoring vessel e.g. about 15 sec., the short-lived corrosive species 15 sec., in the solvent will have decomposed, and the solvent will have cooled to below steam generator temperatures. Conditions in the corrosion monitoring vessel will therefore not be representative of steam generator conditions, and any corrosion measurements made in this vessel will therefore be unreliable.
Accordingly, it is a continuing problem in this art to provide monitoring equipment that provides an indication of corrosion during chemical cleaning and that operates in conjunction with the steam generator cleaning at an earlier time in the cool down cycle.
It is an additional continuing problem in this art to simulate corrosive conditions accurately during steam generator cleaning.
It is still a continuing problem to provide automated corrosion monitoring equipment for use during steam generator chemical cleaning.